The present invention relates to a driving circuit for organic light emitting displays (OLEDs), and more specifically, to a driving circuit applied to drive organic light emitting diodes and amorphous silicon thin film transistors (a-Si TFT) in a unit pixel to prolong lifetime of the OLEDs.
With the advance of techniques for manufacturing integrated circuits, the development and progress of electronic science cause various electronic products fabricated with digital and complicated designs. And for the conveniences of portability and utility, these electronic products are designed with smaller appearances, multiple functions and rapid processing rates. Thus, the products of new generation are easy to carry and fit modern life. Especially because the powerful processing ability of multimedia products can handle easily the audio, visual and graphical digital data, the visual displays are widely researched and developed. No matter what kind of electronics, such as PDAs, laptops, walkmans, digital cameras or mobile phones, all need the display panels for viewing and browsing.
In conventional manufacturing processes of displays, because the techniques of thin film transistors are mature, the liquid crystal displays with the advantages of lightweight, lower consumption and non-irradiation are favored and widely used by consumers. However, with the research and development of organic light emitting diodes, the new generation of organic light emitting displays have further advantages of high light-emitting efficiency, high responding rate, power saving, no limitation of viewing angle, lightweight, thinness, brightness and all colors. And by applying the OLEDs the portable electronic products are manufactured with smaller sizes and finest graphic displaying effects.
Please refer to FIG. 1, a circuit 10 of unit pixel for OLEDs in the prior art is illustrated. The circuit 10 is defined on an amorphous silicon substrate and has two thin film transistors 12, 14 and a storage capacitor 16 so as to drive an organic light emitting diode 18. The transistor 12 is briefly served as a switch device of which a drain electrode is connected to a data line, a gate electrode is connected to a scan line and a source electrode is connected to both one terminal of the storage capacitor 16 and the gate electrode. On the other hand, a drain electrode of the transistor 14 is connected to a power line (Vdd). And a source electrode of the transistor 14 and another terminal of the storage capacitor 16 are both connected to a positive terminal of the organic light emitting diode 18. With regard to a negative terminal of the organic light emitting diode 18 is connected to a power line (Vss).
Thus, the signal from the scan line can turn the transistor 12 on to transfer image data of the data line to the unit pixel. When the transistor 12 is turned on, the data signal on the data line can transfer to the gate of the transistor 14 and be stored in the capacitor 16. This data signal can also turn the transistor 14 on to transfer the voltage signal of the power line (Vdd) to the positive terminal of the organic light emitting diode 18 for luminescence. The data voltage stored in the capacitor 16 can be applied to keep the transistor 14 turned on while the signal on the scan line turns the transistor 12 off so as to maintain the organic light emitting diode 18 at a certain current level.
However, it is noted that in the above circuit design, the voltage difference VOLED between two terminals of the organic light emitting diode 18 will affect the gate-to-source voltage (Vgs) and the drain current (Id) of the transistor 14 due to the organic light emitting diode 18 is connected directly to the source electrode of the transistor 14. The current formula is shown as follows:       Id    =                            1          2                *                              K            ⁡                          (                              Vgs                -                Vth                            )                                2                    ⁢              
            ⁢              xe2x80x83            =                        1          2                *                              K            ⁡                          [                                                V                  ⁢                  data                                -                                  (                                                            V                      OLED                                        -                                          V                      SS                                                        )                                -                Vth                            ]                                2                      ,
In above formula, K is a constant, Vdata is the voltage signal on the data line, and Vth is the threshold voltage of the transistor 14. After a long time of operation, the voltage difference VOLED between two terminals of the organic light emitting diode 18 will increase so as to reduce the drain current (Id), to decrease the lightness of the organic light emitting diode and to shorten the lifetime of the displays.
A purpose of the present invention is to provide a unit pixel circuit for OLEDs to prevent the voltage difference between two terminals of the organic light emitting diode from varying and to avoid of reducing the operating current of the driving transistor.
Another purpose of the present invention is to provide a circuit design to prevent from reducing the brightness of the OLEDs and to prolong the lifetime of the displays.
The present invention discloses a driving circuit of an organic light emitting diode. The driving circuit comprises the components as follows. A driving transistor has a gate, a source and a drain, wherein the drain is connected to a power line and the source is connected to the organic light emitting diode. A first switch transistor has a first gate, a first drain and a first source, wherein the first gate is connected to a scan line, the first source is connected to the power line, and the first drain is connected to the gate of the driving transistor. When the first switch transistor is turned on by the scan signal on the scan line, the voltage signal on the power line will turn the driving transistor on. A second switch transistor has a second gate, a second drain and a second source, wherein the second gate is connected to the scan line, the second drain is connected to a data line and the second source is connected to the source of the driving transistor. When the scan signal on the scan line turn the second switch transistor on, the data signal on the data line can transfer to the source of the driving transistor.